The invention relates generally to a method of rapid prototyping and manufacturing and, more particularly, to laser sintering and the ability to prevent the build up of monomers and oligomers that condense and solidify in the form of films or larger crystals on windows within the process chamber during laser sintering operations.
Rapid prototyping and manufacturing (RP&M) is the name given to a field of technologies that can be used to form three-dimensional objects rapidly and automatically from computer data representing the objects. In general, rapid prototyping and manufacturing techniques build three-dimensional objects, layer-by-layer, from a working medium utilizing sliced data sets representing cross-sections of the object to be formed. Typically an object representation is initially provided by a Computer Aided Design (CAD) system. RP&M techniques are sometimes referred to as solid imaging and include stereolithography, ink jet printing as applied to solid imaging, and laser sintering.
A laser sintering apparatus dispenses a thin layer of heat-fusible powder, often a fusible polymer powder, polymer coated metal, or ceramic, across a process chamber to create a bed of the powder. The laser sintering apparatus then applies thermal energy to melt those portions of the powder layer corresponding to a cross-section of the article being built in that powder layer. The article is formed within a mass of powder commonly referred to as the “part cake.” Lasers typically supply the thermal energy through modulation and precise directional control to a targeted area of the powder layer. Conventional selective laser sintering systems, such as the Vanguard™ system available from 3D Systems, Inc., use carbon dioxide lasers and position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam.
The part cake is supported on a moveable build platform upon which the bed of powder is disposed. After a powder layer is fused, the build platform moves downward by an incremental distance and the apparatus then dispenses across the powder bed an additional layer of powder onto the previously fused layer and repeats the process of melting and selective fusing of the powder in this next layer. Fused portions of later layers fuse to fused portions of previous layers as appropriate for the article, until the article is complete. These articles are sometimes referred to as “built parts.” Each additional layer of powder is typically dispensed from a powder feed system that dispenses a measured amount of powder onto the powder bed. A powder spreader, such as a blade or roller then picks up and spreads the powder over the powder bed in a uniform manner.
Detailed descriptions of laser sintering technology may be found in U.S. Pat. Nos. 4,863,538; 5,132,143; and 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 to Housholder.
Generally, once the part is built it remains within the process chamber under an inert atmosphere until the newly formed part has cooled. Cooling may require several hours or days, depending upon the size and number of parts being built in a single build.
The most common powder material utilized in laser sintering systems is a nylon polyamide. Because the laser sintering process is a thermally based process the heat generating laser used to fuse the powder material together generates chemical by-products during the process. These by-products are volatile monomers and oligomers that vaporize and condense on the cooler surfaces on the interior of the process chamber. Especially susceptible to the build-up of these chemical by-products are the process functional equipment glass windows on the interior and ceiling of the process chamber. The build-up can be in the form of a thin film or frost-like crystals. The build-up of chemical by-products on process chamber exposed surface of the laser window is especially problematic because over a relatively short period of time the laser power delivered to the powder bed can be attenuated by as much as 50% in areas with film build-up and by as much as 71% in areas with frost-like crystalline build-up. Other process equipment functional windows within the process chamber that can become obstructed with chemical by-product build-up include the IR sensor window and IR camera window for which any attenuation of the IR signal between the powder part bed and the sensing devices can cause catastrophic loss of thermal control during the laser sintering process. If a video camera is employed to film the build process, that process functional window can also become obstructed and prevent effective filming from occurring over time.
This chemical by-product build-up has been long recognized as a problem by laser sintering system manufacturers and users. As early as 1994 DTM Corporation installed a heated nitrogen flow from a plurality of small orifices about the circular laser window ring across the laser and IR sensor windows in an unsuccessful attempt to stop this build-up of chemical by-products, especially in laser sintering systems using nylon polyamide powder material. U.S. Pat. No. 5,876,767 describes a laser sintering apparatus having a radial nozzle outlet orifice about the entire laser window ring that discharges a stream of nitrogen to stroke across the entire image-side surface of the circular laser window lens radially inwardly in an attempt to prevent monomers produced when solidifying the powder from precipitating on the lens. This approach encountered the same problems as the earlier DTM Corporation approach. Both approaches were further hampered by the fact that the relatively high flow rate of inert gas. In both prior approaches the inert gas flows create turbulence so that powder dust particles that float within the process chamber are entrained in the inert gas flow and adhere to the process chamber exposed surface of the windows when the gas flow impacts the window or lens.
Thus, there exists a need for an effective approach in a laser sintering apparatus to minimize the build-up of chemical by-products on the process chamber exposed surfaces of process equipment functional windows. These problems are solved in the design of the present invention.